Extruded foam products have been known for a long time. Initially, combustible propellants were used in the manufacture of such products. An early breakthrough in the production of such foams was the use of chlorofluorohydrocarbons (CFC) as the propellants instead of the combustible propellants. Pentane had also been used as propellant. Once CFC compounds trichlorofluoromethane, dichlorodifluoromethane, trichlorotrifluoroethane and dichlorotetrafluoroethane started to be used in foam extrusion processes, semi-finished articles and products could be produced which met the market demands in respect of both quality and operational use.
However, it has recently been ascertained that CFC gases, which are heavily used in aerosol and refrigeration technology and in the electronics industry, substantially accelerate the destruction of the ozone layer in the upper atmosphere of the earth. Accordingly, there has recently been a return to the use of pentane as a propellant because, although it is slightly inflammable, it does not damage the ozone layer.
In general, the extrusion devices which are to be used to produce foams do not differ substantially from conventional extrusion devices. However, metering systems for the addition of the propellant must be used to ensure that the propellant cannot escape due to wear in the pressure build-up system of the pump for the propellant. If such escape occurred, the risk of fire would be considerably increased. In addition, it is advisable for the propellant metering systems to be separate from the extrusion device. Further, safety measures which are essential are intensive ventilation in the form of air renewal in the location where the foam extrusion device is installed, in the intermediate film layer, in the deep-drawing section and in the section where the foam waste is re-extruded. It is also necessary to provide means for removing electrostatic charge from the semi-finished foam articles and products.
In consequence, specific demands are made of the propellant when, particularly, foamed polystyrene is being produced by extrusion. The propellant must be soluble in the fused mass of polystyrene, but its viscosity and its glass point must not be significantly changed. During the foaming, the propellant must have a high rate of evaporation, so that the concentration of residual gas remains minimal. No permeation of the propellant can be permitted because this would cause the foamed cells to collapse before they had time to harden. Propellants which have these properties and have been used in practice include pentane, trichlorofluoromethane, dichlorodifluoromethane and, in exceptional cases, nitrogen.
Nitrogen can be successfully used as the propellant when a sufficiently high concentration of solution can be achieved. However, in prior art systems, this could only be achieved by providing a large quantity of metering machines, which is not feasible. The slight affinity of nitrogen for the fused mass of polystyrene necessitates the use of a very high solution pressure, which is substantially higher than that necessary if dichlorodifluoromethane is used.